Snowboarding has a need for special motions and physical requirements not discovered in other sports including skiing. A person who rides a snowboard, i.e. snowboarder should stand on the snowboard only with his/her feet by use of devices used to safely attach the feet onto the snowboard. However, the attachment devices have a significant limit in strength and installation position thereof. It is noted that, when descending a snow-covered slope by snowboarding, a motive force to drive the snowboard is gravity. Therefore, the snowboarder needs to carefully guess the position of his/her body differently from other sports. In particular, it is noted that an angle between the center axis of each foot and the center axis of a snowboard greatly varies according to different snowboarders and various snowboarding tricks and descent techniques. Also, a boot set into a binding mounted on the snowboard is often used as a device to support the lower part of the leg right above the ankle. However, if the ankle at the center of the leg is changed in position relative to the snowboard, it may cause a change in an angle between the leg and the foot. Currently, there has been generally used a pair of rigid supporting devices vertically erected from a snowboard and aligned, respectively, with the center axes of both feet. The supporting devices are designed to be folded down onto a surface of the snowboard. Meanwhile, to assure a freedom in the implementation of various snowboarding motions, it is necessary to appropriately control the dismantling and remounting of bindings to and from a snowboard. Also, a distance between two bindings attached onto the snowboard and relative positions of the bindings on the basis of a longitudinal length of the snowboard should be changed according to snowboarders having different body sizes.